The present invention relates to vehicle wheel alignment, and more particularly to vehicle wheel alignment systems for use with vehicles which utilize shims to simultaneously adjust the camber and caster of a steerable wheel on a vehicle.
U.S. Pat. No. 4,381,548 to Grossman and January describes a wheel alignment system which, inter alia, performs computations to determine the shims required to adjust caster and camber. That patent states:
"The inboard mounting points of the upper control arms are adjusted by washer-like shims. Changing equal shims at both mounting points changes camber, but not caster, while adding shims at one mounting point while removing equal shims from the other, changes caster but not camber. The shims are approximately calculated such that a one-sixteenth (1/16) inch thick shim change, equal at both mounting points of the control arm, produces a one-fourth (1/4) degree camber change; and a one-thirty second (1/32) inch thick shim change, equal and opposite at both mounting points of the control arm, produces a one-half (1/2) degree caster change. From the camber and caster angles measured and the specifications entered, the computer 13 uses the above calculations to compute the net shim changes required."
However, as noted in the '548 patent, the computations are ". . . especially for General Motors type vehicles which use this shim method of adjustment." Large General Motors automobiles have used this method of adjusting camber and caster for many decades. General Motors pickup trucks use the method also, but have the control arm mounted "outboard" the frame while the automobile usually has it mounted "inboard" the frame, and so require shim removal where the automobiles require shim additions. In recent years, a smaller percentage of General Motors automobiles use this method of adjusting camber and caster, while an increasing percentage of small pickup trucks use it. It is especially common with imported trucks.
The wheel alignment system described in the '548 patent is of reduced benefit when aligning most vehicles which use front shims to adjust camber and caster, because the computations performed are only approximately correct. These vehicles differ in one or more of the four following ways when compared to the large GM automobiles for which the system was designed: (1) the control arms have different shapes; (2) the control arms have different dimensions; (3) the control arms may have a different orientation relative to the frame; and (4) the control arms may be mounted outboard the frame rather than inboard. Any of these differences is sufficient to make the approximation of the '548 system unacceptably inaccurate in computing the required shims to correct camber and/or caster.